The U.S. Pat. No. 4,662,217 a method and an apparatus for determining tilting and rolling moments acting on a vehicle in a wind tunnel comprising four force-introduction plates each of which supports a wheel of the vehicle. Each force-introduction plate is associated with a respective guide member guiding a respective force-transmission member in a vertical direction, the guide members being connected together by a rigid frame. Each force-transmission member is connected to a respective force-introduction plate and acts on a respective force-measuring device. In a first object-related mode of operation, known as a 7-components configuration, the guide members are attached to the base, and the force-transmission members are moveable relative to the guide members. Wheel loads are measured and moments can be calculated using the distances between the wheels. In a second apparatus-related mode of operation, known as a 6-components configuration, the guide members are released from the base and attached to the force transmission members. Moments are obtained from force measurements and the distances between the force-transmission members. Hence the influences of a displacement of the force introduction points may be reduced; however, upon application of forces onto the vehicle, as by applying an airflow, a displacement of the car body in respect of the chassis comprising the motor and the wheels may occur, which displacement results in a displacement of the actual center of gravity in turn resulting in an error in of the readings of the measurement.
Specifically, wind tunnel apparatuses of the type defined above should have a desired accuracy of measurement in the order of 0.25 per mille. However, as indicated before, actual vehicles are displaced in an airflow due to the elasticity of the wheels and the suspensions thereof. Thus, with air velocities in the wind tunnel up to 200 km/h the displacement of the center of gravity of the vehicle in X-direction may be 2 cms.
Such a displacement of 2 cms and a vehicle mass of 1500 kgs would result in an aerodynamic moment about the axis MY of about 300 N.m with the permitted error of measurement of about 0.2 N.m.
Such an error must be expected both for the 6-component configuration and the 7-component configuration, the influence being smaller with the 7-components configuration since in the airflow the vehicle rolls on its tires to the rear which rolling effect at least partially compensates the error moment resulting from the displacement of the center of gravity of the vehicle.
When evaluating the readings of the measurements this error due to the displacement of the vehicle may be corrected by determining the displacement of the car body during measurement in X-direction and Y-direction and introducing these values in the evaluation (see DE-Z ATZ automobiltechnische Zeitschrift 87 (1985) No. 1, pages 31 to 36).
With such a method the displacement of the car body in the airflow is measured, but not the actual displacement of the center of gravity of the vehicle since the wheel suspensions, the motor, the gearing and the car body of the vehicle are displaced non-uniformly in relation the each other resulting in a relative displacement of the center of gravity of the vehicle with respect to the car body. With the known method the displacement of the center of gravity is assumed in a first approximation to be linear. However, such an estimate is very rough; thus, the required accuracy is not maintained.
According to another aspect of the invention the apparatus disclosed in the above mentioned DE-Z ATZ-automobiltechnische Zeitschrift 87(1985), No. 1, pages 31 to 36 uses four separate force-introduction plates for supporting the wheels of the vehicle. It is indicated that for determining the aerodynamic characteristics the measurement of the forces for the four individual wheels is necessary which determination is remarkably affected by the alignment of the wheels and the spring characteristics of the vehicle. Furthermore, errors arise which are directly dependent on the distances apart of the axles of the vehicle and of the wheels on each axle. The article comes to the conclusion that apparatuses using four individual force-introduction plates do not permit a meaningful error correction due to non-controllable tire deformations.
Such errors are caused by sinking and lifting forces acting on the vehicle on the street due to the airflow. Furthermore, in this situation the vehicle undergoes minor oscillations in vertical direction due to the spring system and the elastic suspensions. These vertical osciallations result in changes of the above mentioned distances apart of the wheels and the axles.
However, in a wind tunnel the wheels are fixedly adjusted on the individual force-introduction plates. Thus, the suspension of the chassis is unable to follow any oscillation in the manner as explained above to values of the distances apart in normal traffic. Therefore, there is a considerable difference between the results of measurement in a wind tunnel as compared with the actual situation in normal traffic.